In buildings it is generally desirable to provide an exchange of air such that air from inside the building is expelled and replaced with fresh air from outside the building. In colder climates where the inside of the building is much warmer than the outside air (‘heating applications’) or in hot climates where the inside of the building is air-conditioned and is much cooler than the outside air (‘cooling applications’) there is an energy cost to this. In heating applications the fresh air is typically both colder and drier than the air inside the building. Energy is required to heat and humidify the fresh air. The amount of energy required can be reduced by transferring heat and moisture from the outgoing air to incoming air. In cooling applications the fresh air is typically both warmer and more moist than the air inside the building. Energy is required to cool and dehumidify the fresh air. The amount of energy required for heating and cooling applications can be reduced by transferring heat and moisture between the outgoing air and the incoming air. This may be done using an ERV system comprising membranes which separate flows of incoming and outgoing air. The characteristics of the membranes are an important factor in the performance of an ERV system.
Ideally a membrane in an ERV system should be: air-impermeable such that the membrane can maintain effective separation of the incoming and outgoing air flows; have a high thermal conductance for effective heat transfer between the incoming and outgoing air flows; and provide high water vapor transport for effective transfer of moisture between the incoming and outgoing air flows. Achieving these characteristics typically favors the use of thin membranes.
In addition to the above it is desirable that the membranes be robust enough for commercial use, cost effective to produce, and compliant with any applicable regulations. At least some jurisdictions have regulations that relate to the flammability of membranes used in ERV systems. For example, UL 94 is a standard released by Underwriters Laboratories of the USA which relates to flammability of plastic materials for parts in devices and appliances. UL 94 classifies plastics according to how they burn in various orientations and thicknesses. From lowest (least flame-retardant) to highest (most flame-retardant), the classifications are: HB: slow burning on a horizontal specimen; burning rate <76 mm/min for thickness <3 mm and burning stops before 100 mm; V-2 burning stops within 30 seconds on a vertical specimen; drips of flaming particles are allowed; V-1: burning stops within 30 seconds on a vertical specimen; drips of particles allowed as long as they are not inflamed; V-0: burning stops within 10 seconds on a vertical specimen; drips of particles allowed as long as they are not inflamed; 5VB: burning stops within 60 seconds on a vertical specimen; no drips allowed; plaque specimens may develop a hole; 5VA: burning stops within 60 seconds on a vertical specimen; no drips allowed; plaque specimens may not develop a hole. UL 94 provides additional classifications VTM-0, VTM-1, VTM-2 for thin films. UL 723 is another standard released by Underwriters Laboratories that provides a test for surface burning characteristics of building materials.
One way to make membranes for water vapor transport applications is to apply a thin coating of a thermoplastic polyurethane to a silica-polyethylene substrate. This approach has disadvantage that the substrate does not shrink away from flame. Therefore such membranes may not pass some flammability standards. Also, silica-polyethylene substrates tend to be thicker and less porous than desired. Typical silica-polyethylene substrates have thicknesses >95 microns and porosities of <60%. Thus such substrates result in membranes that offer higher resistance to water vapor transport than is desirable.
Another issue with ERV systems is that in cooling conditions where outside relative humidity and temperature are high, very high latent (moisture) transport is desirable. However, in colder climate conditions in a well-sealed building, high moisture transport may be less desirable as it may cause humidity to build up indoors. Optimal indoor RH is in the range of 30 to 55% to prevent discomfort and also to prevent growth of mold. Some system designers recommend HRVs as opposed to ERVs in ‘heating primary’ climates for this reason. In more extreme heating conditions, some level of moisture transport may be beneficial in the heating conditions prevent low indoor relative humidity, and also to minimize frosting and condensation in the core.
There is a need for membranes suitable for ERV applications and/or other water vapor transport applications that address some or all of these issues.